A vaccinia virus mutant which is resistant to aphidicolin was shown to code for a drug-resistant DNA polymerase. Aphidicolin resistance was transferred to cells infected with wild-type virus by transfection with progressively smaller DNA segments derived from the DNA polymerase gene of the resistant virus. After a 124 bp segment which conferred resistance was cloned in M13 phage, the sequence of one viral strand was obtained and it showed that the mutant had a T replacing a parental G at a single site. Efforts to sequence the other strand by reversing the orientation of the segment were unsuccessful; however, a sequence of the complementary strand was derived from the plasmid containing the mutated segment and it had an A, replacing a C, at the correct site. Site-directed mutagenesis of the wild-type DNA should confer aphidicolin resistance. To facilitate this research, several techniques were developed. One used a chimeric phage-plasmid vector (phagemid), that has two replication origins which allows it to replicate as double-stranded or single-stranded DNA. Production of the latter which is used for sequencing and site-directed mutagenesis, depends on activating the fl origin of the phagemid by helper phage. To utilize phagemids for vaccinia DNA analysis conditions were established for selecting and growing the helper phage and bacteria containing the recombinant phagemids, and for maximizing the amount of single-stranded DNA which was secreted from bacteria. Another development improved the recovery of small DNA segments from agarose gels. A commercial elctrophoresis chamber with a collection well where high salt solutions were supposed to trap electrophoretically transferred DNA, did not work well. Investigation of different solutions showed that the high buoyant density of a concentrated CsCl solution supplemented the effect of the high counter-ion concentration and caused the DNA to concentrate in the collection well. This procedure is probably the simplest and most efficient way to recover small to medium size DNA.